The present invention relates to a pilot operated, control valve and more particularly to such a control valve suitable for effecting two speed operation of a hydraulic jack in a first direction as well as providing for operation of the jack in the opposite direction.
Such control valve has numerous applications, particularly for regulating the operation of hydraulic jacks associated with implements on various material handling or earth moving machines. For example, the control valve is particularly contemplated for regulating a double-acting hydraulic jack coupled with an ejector in the bowl of an earth moving scraper. In this application, the ejector is movable forward through the bowl to unload material from the scraper. Two speed operation is desirable in order to rapidly sweep the ejector through the bowl when it encounters only a light load as well as to provide for lower speed, higher pressure operation of the ejector for removing heavy loads from the scraper. The jack may also be operable in the opposite direction in order to return the ejector to its normal loading position at the rear of the bowl.
Many types of control valves are available in the prior art for use in applications similar to that contemplated by the present invention. Initially, it is noted that many control valves are available to provide for two speed operation of a hydraulic motor or jack. In this regard, reference is made to U.S. Pat. No. 3,170,379 wherein a specially configured jack is responsive to two different control valves for accomplishing two speed operation. Similarly, U.S. Pat. No. 3,227,050, assigned to the assignee of the present invention, contemplates a control valve for effecting two speed operation of a hydraulic jack designed to similarly position an ejector within a scraper bowl.
Within a conventional hydraulic jack of a type having a cylinder and a piston with a connected rod penetrating one end of the cylinder, it is common to take advantage of the differential effective surface area on the piston for accomplishing two speed operation of the jack. For example, one operating speed may be established by communicating the larger effective surface area of the piston in the head end of the jack with a pump while communicating the rod end of the jack with a drain. Such a mode of operation provides for relatively low operating speed of the jack with relatively high operating force applied to the piston in the head end of the cylinder. For higher speed operation, the rod end of the jack may also be placed in communication with the pump. The jack still continues to operate in extension because of the differentially larger effective surface area in the head end of the cylinder. At the same time, exhaust fluid from the rod end of the jack provides make-up fluid for the head end of the jack. This permits the jack to be extended at a higher speed but with effectively reduced operating force in the head end of the cylinder. Such a combination is contemplated for example by U.S. Pat. No. 2,619,118 and 3,633,461.
It has been discovered, however, that most control valves providing for such two speed operation of a jack are of relatively complex design. In addition, such control valves do not always provide for positive operator control over two speed operation of the jack. For example, two speed operation of the jack by the control valve may depend upon operating pressures within the system.